A method of making large articles from ductile iron alloys is disclosed, and, more particularly, a method of making a main shaft of a wind turbine from ductile iron alloys by forming the alloys followed by austempering.
Various alloys have been considered and used to produce large shafts, such as the main shaft of a wind turbine. As an example, forged steel alloys are commonly used for main shafts in wind turbine applications. While forged steel wind turbine shafts are useful, they are generally costly due to their size and the mechanical processing (e.g., forging), heat treating and machining required to obtain the necessary alloy properties.
Due to their strength, toughness, and machinability, ductile iron (cast nodular iron) alloys have also been used to produce wind turbine main shafts. Typical production methods for these shafts have included conventional sand casting. These methods have generally been employed to produce relatively smaller shafts having wall section thicknesses of less than fifteen centimeters. While these methods are useful to produce ductile iron wind turbine shafts, they are generally not well-suited to the demanding service requirements of more recent wind turbine designs, which include larger sizes, including higher casting weights, larger diameters and larger shaft wall section thicknesses. As larger main shafts become required for wind turbine applications, their design requirements for strength and toughness have exceeded the capability of conventional cast and austempered ductile iron alloys, and methods that enable the manufacture ductile iron main wind turbine shafts having larger sizes, including higher casting weights, larger diameters and larger shaft wall section thicknesses is very desirable.